This invention relates in general to photocells and the processes of their manufacture and relates more particularly to a class of materials, photocell structures compatible with these materials and associated manufacturing processes that produce high efficiency, inexpensive photocells.
In the figures, each element indicated by a reference numeral will be indicated by the same reference numeral in every figure in which that element appears. The first two digits of any 4 digit reference numerals and the first digit of any two or three digit reference numerals indicates the first figure in which its associated element is presented.
The oil embargo of the late 1970""s sensitized the world to the problem of limited petrochemicals in the world and the concentration of such chemicals in several regions around the world that are unstable economically and politically. This produced a step increase in the interest level for using renewable energy sources, such as solar power, wind power and tidal power. The recent war between the United States and Iraq has reconfirmed the need for a stable energy source that is not affected by political events around the world. In addition, the desire for clean air is so acute, that interest in nuclear power may be revived despite the well-known radiation dangers and lack of nuclear waste treatment methods. Unfortunately, the progress in developing alternate energy sources has been disappointing and has shown that the development of such technologies is very difficult.
Although there was initially a high level of hope that high efficiency, photovoltaic cells could be manufactured to produce directly from incident solar energy the large amounts of electricity utilized throughout the world, the photovoltaic cells produced up to now have been commercially viable only for special niche markets, such as: solar powered calculators for which consumers are willing to pay the additional cost to avoid the problems of battery replacement; solar powered telephones for use in areas that are remote from electrical power lines; and buildings located in regions of the country that are sunny and sufficiently remote from commercial power lines that solar power is a cost effective alternative. If solar energy is to provide a significant fraction of this country""s or the world""s power needs, the average cost per Watt for solar photovoltaic cells over the life of such cells must be reduced to a level that is competitive with the average cost per Watt of power from existing electrical utilities over the same period.
At the present time, the average cost per Watt of photovoltaic power, over the life of the photovoltaic cells, is more than five times the typical cost per Watt of electricity produced by present day electric power plants. It is therefore necessary to greatly reduce the cost of photovoltaic cells in order to reduce both the purchase price of a photocell array and the average cost of electricity produced by such cells over the useful life of that array. For solar electric energy to be practical for use by electrical power utilities or to be a practical alternative for use by electrical power consumers, a photocell design must be provided that: has a low material cost; has a highly efficient structure; and can be manufactured in large volumes by low-cost manufacturing processes. The design of this photocell requires an interactive analysis of materials, cell structure and fabrication processes. To produce low cost, efficient cells in the volumes needed to supply a significant fraction of the world""s power needs, the manufacturing processes must provide high deposition rates and high layer uniformity over a large area photocell.
A first significant factor in the manufacturing cost of a solar photovoltaic cell is the cost of the materials needed to manufacture this cell. The cost of the material in the photosensitive, current-generating layer of the photovoltaic cell can be a significant fraction of the cost of manufacturing such photovoltaic cell. To efficiently convert incident radiation, this layer must convert most of the incident solar energy into electrical power. If the absorbance value of the photosensitive, current-generating material is small, then its thickness must be correspondingly large to absorb and convert most of the incident solar energy. Because many photosensitive, current-generating materials are relatively expensive, a significantly increased thickness of this layer can significantly increase the total cost of a photovoltaic cell utilizing that material.
Even when such low photosensitivity material is not expensive, it can still significantly increase the cost of the photovoltaic cell. The increased thickness of the photosensitive, current-generating layer increases the average pathlength that the photovoltaically generated charged species must travel to corresponding electrodes. This produces a concomitant increase in the electrical resistance of such layer, thereby decreasing power conversion efficiency. In order to avoid unduly degrading the amount of electrical power produced for a given flux of incident solar energy, the photosensitive, current-generating layer must have a high level of purity in order to have a high enough conductivity that resistive losses do not significantly degrade performance. Such increased material purity requirements can greatly increase the cost of such solar energy cells.
Much of the research and development of solar cells has been directed toward single crystal silicon photocells, because the tremendous amount of knowledge about solid-state circuits manufactured in a silicon substrate can then be applied to this problem. Silicon also has the advantage of being a non-toxic, readily available resource. However, crystalline silicon is a relatively poor solar absorber, because it is an indirect bandgap material. This means that a relatively thick crystalline layer must be utilized to absorb a significant fraction of the incident solar energy. Unfortunately, this increased thickness will degrade efficiency, because of the concomitant increase in the resistance across which the photogenerated charge carriers need to travel. This increase in resistance because of this increased thickness must be offset by a reduction in resistivity by the use of high purity, high perfection silicon layers. Such layers are very costly and therefore significantly increase the cost of single crystal silicon photocells.
These thick layers of silicon must be made by the expensive process of solidification from the melt in a single crystal boule that is then sliced to form the crystalline wafer. Approximately half of this crystal is lost during this slicing process, further increasing the cost. Even though the silicon photocells are durable and efficient, their cost is still prohibitively high for utility power. Although the conventional single crystal layer growth process can be modified to produce lower cost, polycrystalline photocells, this change in the material structure also reduces the efficiency of the resulting photocell, such that the resulting cost of electric power is still too high to compete with existing electrical utilities.
Amorphous silicon is attractive for use as the photosensitive, current-generating layer in photocells, because its high absorptivity for solar energy enables the photosensitive, current-generating layer to be extremely thin, thereby reducing the material cost of that layer and reducing the resistive losses of that layer. This amorphous silicon layer is also very insensitive to impurities. This results in a very inexpensive layer that, unfortunately, due to the nature of electricity transport in amorphous materials, has a very low efficiency.
Although the efficiency can be increased by producing several amorphous layers in a stacked arrangement, this also increases the cost enough that the resulting device is not commercially competitive. Amorphous silicon, which is actually an alloy of hydrogen and silicon, also has a more serious weakness that, when exposed to sunlight, hydrogen is gradually liberated, thereby severely degrading the efficiency of the device. The lifetime of such photocells is too short to collect enough electricity to pay for their cost. In addition, the production of this material is difficult, because, at the low substrate temperature required for the growth of this amorphous phase, the growth rate is low and the source chemicals are not fully dissociated. This significantly increases the cost of this material.
For the above reasons, it was important to search for alternative materials for use in solar photovoltaic cells. Gallium arsenide (GaAs) and aluminum gallium arsenide (AlXGa1xe2x88x92XAs) have been investigated, developed and utilized for use as solar cells. These materials have been used to make the most efficient solar cells yet made. Unfortunately, the cost of these devices is more than ten times the cost of silicon devices, so that these devices are utilized only when the cost of such devices is not a significant factor. Although these devices are used for space power and high performance solar electric race cars, they are unsuitable for electric utility power. In addition, these materials contain gallium, which has limited availability and contain arsenic which is both a poison and a carcinogen. The use of this material on a scale suitable for producing a significant fraction of the electrical power needs of the U.S. or the world would create tremendous environmental problems. Indeed, the tremendous volume of photovoltaic cells that must be manufactured to provide the ability to generate a significant fraction of our energy needs, means that every choice of material in such photovoltaic cells must be evaluated as to the resulting impact on the cost of materials needed to manufacture such devices and the resulting impact on the environment of manufacturing and/or disposing of such a tremendous volume of these photovoltaic cells.
Cadmium telluride (CdTe) has been actively developed for solar electric power for many years. This material has achieved high efficiency in small area devices and research continues toward obtaining high efficiency over large areas. However, even if the junction efficiency and humidity degradation problems were solved, it would still be inadvisable to use this material for terrestrial solar electric power, because cadmium and tellurium are both dangerous environmental poisons. In addition, tellurium is a rare and expensive material. Such rare and expensive materials should only be considered for use as dopants, because any other use would make the resulting device commercially impractical and would rapidly deplete the available quantities of such materials.
Another material currently being developed for solar electric applications is copper indium diselenide (CuInSe2, called CIS). Small cells of high efficiency have been made but the process used for their growth is complex, costly, nonuniform for large areas, and requires large amounts of the extremely toxic gas hydrogen selenide. Indium is an expensive and very rare chemical element, whose cost and availability have not been a problem to date, because it has been used only as a dopant. This means that only a minute amount of this material is needed in any given device, so that the total demand has not yet significantly depleted the amount of this material that is available. However, if solar electric cells using indium as a primary component were used to produce a significant fraction of the world""s power needs, the cost would rise rapidly as the supply of this rare element became depleted. Another problem with these solar photovoltaic cells is that selenium is both relatively rare and toxic and its widespread use would be inadvisable.
It is therefore necessary to locate materials, for use in the manufacture of solar photovoltaic cells, that are abundantly available so that the cost will be low and the available amount of such material will not be significantly depleted, even at the tremendous volume of solar photovoltaic cells needed to provide a significant fraction of our power needs. These materials should also be nontoxic, so that these volumes will not pollute the environment. It is also important that these materials be readily available from many sources so that there is no possibility of a cartel controlling such resources and producing problems similar to the oil embargo of the 1970""s. These materials must be capable of low cost deposition on large area substrates with high uniformity.
In accordance with the illustrated preferred embodiment, new materials are identified as being appropriate for use in the manufacture of solar photovoltaic cells that are sufficiently efficient, inexpensive and durable that they can competitively supply a significant fraction of the world""s electric power needs. These materials were identified by considering the following factors. The chemical elements from which the material is formed must all be inexpensive and abundantly available, so that the huge volumes required will not deplete the resources of such materials or increase the cost of such materials to levels that would prohibit widespread use of these devices. These elements should be available throughout the world, so that a cartel cannot interfere with reasonable, inexpensive access to these materials.
These elements must form a semiconductor material, so that a photovoltaic diode device structure can be formed to convert sunlight to electricity. This semiconducting material must absorb sunlight efficiently in a very thin layer, so that only a small amount of photovoltaic material is required for a large area device and so that the photogenerated carriers are required to travel only a short distance before being collected by the diode junction. This latter benefit enables the use of lower cost, lower purity materials than would be necessary in a thick photosensitive, current-generating layer to keep recombination losses low, thereby reducing material costs of the photocell. The material should also be able to be deposited in a thin film form using processes similar to integrated circuit techniques now in use, so that the expertise in these fields and the manufacturing chemicals, equipment and designs can be applied to the manufacture of these solar cells.
Semiconductors have a threshold energy for the absorption of incident photons, known as the xe2x80x9cbandgapxe2x80x9d, which is characteristic of that semiconducting material. A photon that has an energy higher than the bandgap will be strongly absorbed, whereas a photon that has an energy lower than the bandgap will not be strongly absorbed. Therefore, the photovoltaic semiconductor must have a bandgap that is matched to the incident solar spectrum. If the bandgap is too high, then fewer of the available photons will be absorbed, which reduces the device efficiency. If the bandgap is too low, then the voltage of the device (which is proportional to the bandgap) will be low, which reduces the device efficiency.
The spectral distribution of the solar energy incident on the earth outside of the earth""s atmosphere differs somewhat from the spectral distribution of the solar energy at the earth""s surface. This difference arises because of the absorbance and reflectance of the earth""s atmosphere. Therefore, the optimum bandgap of the photovoltaic material differs according to whether the solar cells are to be utilized above the earth""s atmosphere or at the earth""s surface. As a practical matter, these two distributions are sufficiently similar that there is negligible impact on the choices of materials to be utilized for the photovoltaic layer. Several independent studies have shown that efficient solar cells can be made from a semiconductor that has a bandgap between 1 and 2 electron volts, with the optimum being approximately 1.5 electron volts.
Semiconductors which have a sharp transition in absorption at the bandgap are known as xe2x80x9cdirectxe2x80x9d bandgap materials, and those which have more of a slope at the transitions are known as xe2x80x9cindirectxe2x80x9d bandgap materials. The ideal photovoltaic materials have a xe2x80x9cdirectxe2x80x9d bandgap, because such materials absorb incident light in a very thin layer (less than one micrometer thick), whereas an xe2x80x9cindirectxe2x80x9d bandgap material requires a thick layer (over one hundred microns thick) to absorb the same fraction of incident light. Devices based upon a thin layer of direct bandgap material are less expensive, because less material is needed and the photovoltaic layer can be vapor deposited as a thin film.
It is difficult to deposit low cost, high quality, thick layers from a vapor, so xe2x80x9cindirectxe2x80x9d bandgap materials, such as silicon, are manufactured by solidification from a liquid melt and then sawed into wafers for use as devices. This melt growth process is very expensive and wasteful due to loss of material during cutting. In addition, the resulting discrete devices (typically less than 150 millimeters across) require additional costly assembly into larger modules for final installation. The use of thin films also reduces the distance that the generated carriers must traverse before being collected. This reduces the mobility and lifetime requirements for the current carriers in this layer, which enables high efficiency devices to be formed from a relatively lower quality layer. This further reduces the cost of high efficiency devices. However, indirect materials have the advantage of longer lifetimes of generated carriers that can produce highly efficient devices if the purity and perfection of the thick film can be obtained. Therefore, while indirect bandgap materials are less likely candidates, they should also be considered if a low cost of the thick layer is possible.
To be commercially viable for generating a significant fraction of the electrical power needs of the United States at this time, the resulting photovoltaic cell should maintain an efficiency of at least 15% for a period of greater than 30 years and must be made of materials selected such that the installation of more than 20 billion square meters of cells in less than 20 years will not significantly deplete the supply or inflate the cost of these materials. The total cost of these cells should be on the order of, or less than, $50/m2, so that the average cost of electricity from these solar photovoltaic cells over the lifetime of these cells is comparable to the projected cost of generating that electricity from conventional sources.
The following five materials satisfy all of the these requirements and therefore will produce photovoltaic cells having high efficiency at greatly reduced cost: monoclinic zinc diphosphide (also referred to as beta zinc diphosphide and indicated by xcex2-ZnP2); copper diphosphide (CuP2); magnesium tetraphosphide (MgP4); xcex3-iron tetraphosphide (xcex3-FeP4) and mixed crystals formed from these four materials.
Large-scale solar photovoltaic cells will typically be manufactured as arrays of smaller cells. In order to produce a significant fraction of our national electric power needs (on the order of 3,000 billion kWh/year), the total area of these cells must be on the order of 15,000 square kilometers. In order to manufacture such a tremendous area of photovoltaic cells, it is advantageous to utilize a reactor design that can manufacture sheets of photovoltaic material having a width on the order of a meter or more. Processing sheets of this size requires processes that produce highly uniform thin films over this entire width. And this process must operate near, or below, atmospheric pressure. If the pressure were significantly above one atmosphere, the walls of the equipment used for the process would have to be very thick and heavy, the inherent difficulty and danger in such a process would increase the effective cost of the cells produced.
All of the proposed materials are compounds that contain more atoms of phosphorus than atoms of the metal species. They are all known to decompose to compounds having less phosphorus content, unless they are maintained in an atmosphere of phosphorus gas whenever they are exposed during heating. These materials must be deposited on a heated substrate during deposition of a thin film in order to form a layer having the properties that approach those of single crystals, as required for high efficiency solar devices. It is commonly believed that crystals of these materials must be grown at pressures from 3-10 atmospheres. The existing synthesis literature reports very high pressure crystal growth under phosphorus gas overpressures. These pressures are prohibitive for the growth of large area thin films using conventional atmospheric pressure, or vacuum, equipment.
The existing literature on the growth of xcex2-ZnP2 crystals shows growth occurring at very high pressures (on the order of several atmospheres) and a large excess of phosphorus to produce zinc diphosphide instead of sesquizinc phosphide (Zn3P2) and therefore indicates that large area substrates cannot be coated with ZnP2. However, a detailed analysis of the thermodynamics of the growth of ZnP2 and the conditions of its decomposition to Zn3P2 has shown that in the temperature range required by the proposed manufacturing process for ZnP2 thin films there exists a region of obtainable partial pressures that enables ZnP2, with nearly single crystal properties, to be manufactured at, or below, atmospheric pressure. This enables the production of potentially highly efficient solar cells based upon ZnP2 using conventional process equipment technology.
Analysis of the available literature on the other materials (CuP2, MgP4, and FeP4) has shown (where the data exists) that they too can be grown with near single crystal properties at conditions obtainable with modification to the conventional process equipment technology.
Thin films of these materials must be produced by some means. Solidification of the liquid phase is not possible due to the high pressures to prevent decomposition of the melt. Vacuum evaporation techniques, while possible, are not useful because of the nonuniformity of the deposit if done over large areas or for long times. Chemical vapor deposition (CVD) is the preferred method of depositing thin films of any of these materials. CVD has the advantages of producing high purity, highly uniform thin films over very large areas with the ability to conform to surface irregularities, allows very high deposition rates and efficient abrupt junctions to be formed. CVD equipment is comparatively inexpensive and can be easily scaled to use very large substrates, especially if an atmospheric pressure process is used. CVD is particularly suited for adaptation to the continuous growth processes preferred for the high throughput required to mass-produce the large amounts of cells needed for solar electric utility power. The cost of layers produced by CVD can be made sufficiently low by the use of low cost source species, a process for growth near equilibrium conditions where a near stoichiometric gas composition can be used (reducing waste of source chemicals) and by proper design of the gas flow system (allowing efficient utilization of the source chemicals).
The CVD method requires a vapor transport species that has a sufficiently high vapor pressure to transport the elements to the substrate without condensing on the walls of the growth apparatus. The metallic elements have transport species (in particular, organometallic molecules) that are useful for this process and are commercially available in high purity. The cost of these species, if manufactured in the large volumes expected to be needed, would be low enough to meet the cost criteria on the resulting solar device.
A new phosphorus source has been identified that significantly reduces the cost of manufacture and yields the high purity layers required by all five above-identified photovoltaic materials. This new phosphorus vapor transport species is liquid white phosphorus and is used in a reactor having walls that are heated to prevent condensation without decomposition. White phosphorus is the form of phosphorus that results from smelting phosphate ores, that is purified to make high purity red phosphorus, and that is used to synthesize organophosphorus compounds. It is the cheapest and highest purity form of phosphorus and can be used directly for CVD in an appropriately designed reactor.
Selection of the phosphorus transport species is a problem for conventional CVD processes. The most commonly utilized gaseous phosphorus source for use in CVD is phosphine (PH3). Unfortunately, it is extremely toxic (one breath at 50 ppm is fatal half of the time) so the entire system must be absolutely leak-tight, all process areas must be fail-safe ventilated and monitored with phosphine detectors, thereby increasing the downtime, facility and maintenance costs. Since it is so toxic, few manufacturers will supply it and the cost of phosphine is very high. In addition, phosphine does not dissociate completely at substrate temperatures as high as 650xc2x0 C. and much of it goes through the reactor without reaction. This increases the effective cost of the process and produces a difficult exhaust treatment and environmental safety problem. This also precludes the use of phosphine in low substrate temperature processes. Although trimethylphosphine and triethylphosphine can be utilized as the phosphorus source, these two species are also quite toxic and they allow incorporation of carbon into the thin films. Bisphosphinoethane and tertiary butyl phosphine have recently been introduced as commercially available phosphorus sources, but these are about fifteen times as expensive as phosphine and therefore contribute significantly to the manufacturing cost of the photovoltaic cells. Research systems designed to produce phosphine in-situ by reacting plasma produced atomic hydrogen with heated red phosphorus have not proved useful in a production enviromnent due to difficulty in controlling the source reaction for uniform repeatable processes.
All of the proposed photovoltaic materials have high solar absorption and low bulk density and, thus, are suitable for the manufacture of efficient lightweight solar cells for use on solar powered airplanes, lighter-than-air craft, satellites and other applications where it is important to have lightweight solar power. Because the weight and cost of the substrate of a solar cell is the dominant fraction of the total weight and total material cost, it is important to use lightweight, low cost substrate materials. Plastic substrate have the advantage of being both light weight and low cost. In order to utilize this class of substrates, the process for depositing the solar cell materials should be at a low enough temperature to be compatible with this choice of substrate. This can be achieved by use of plasma enhancement to the proposed chemical vapor deposition process which would be set to operate at a temperature at, or below, 375 degrees Centigrade. The use of a plasma to excite the reactant species and carrier gas enables the production of high quality films at a lower temperature because part of the energy required for the deposition process is supplied by these excited species as they arrive at the substrate. This low temperature capability enables this class of substrate materials (plastics) to be utilized for the fabrication of solar photovoltaic cells.
In order to produce photovoltaic cells of the highest possible collection efficiency, the purity of the photovoltaic layer should be as high as possible. The use of a plasma to dissociate hydrogen gas into hydrogen atoms can be utilized to reduce the incorporation of carbon (from the organometallic species used to vapor transport the metal species) into the photovoltaic layer by reacting with, and removing, any carbon that becomes exposed during the growth process. This is particularly critical for devices based upon iron phosphide, or mixed crystals containing iron, because the tendency for iron to form a very stable carbide. The plasma can be used to enhance the purity of the deposited thin film and thus the efficiency of the photovoltaic device fabricated from the film.
It has been observed that the surface of xcex2-ZnP2 crystal oxidize slowly on air at room temperature over a long time and this decomposition is accelerated by temperature and in moist environments. This process would occur at a significant rate in a solar cell of standard design (exposed or protected by a thin antireflection coating) under conditions of its normal operation. In particular structures must be used to protect the critical diode junction against such decomposition. This protection is achieved by using a homojunction device where the junction is slightly below the surface of the layer and by the use of a passivating layer that prevents the transport of atmospheric oxygen and moisture to the surface or the transport of the initial decomposition products from the surface, thereby stopping the reaction before the junction region is damaged. By this means, a practical cell lifetime of over 30 years can be achieved. In order for a solar cell having this passivation layer to also have high efficiency it is necessary that the passivating layer also has high electrical conductivity and high transparency to the solar spectrum. There are several materials able to function in this multiple role. A unique and particularly attractive one for this purpose is zinc phosphate which has the required properties, is known to prevent decomposition of ZnP2, and can be produce in the same reactor utilized to produce the ZnP2 layer (possibly using the same source species).